The present invention relates to a method for fabricating a novel electrode for a rechargeable lithium battery.
The battery performance of rechargeable lithium batteries recently under intensive research and development, such as charge-discharge voltage, cycle life characteristics or storage characteristics, depends largely upon the types of the electrodes used. This has led to the attempts to better battery performance by improving electrode active materials.
The use of metallic lithium for the negative active material, although possible to construct a battery with high energy density per weight and volume, presents a problem that the lithium deposited on charge grows into dendrite which might cause internal short-circuiting.
Rechargeable lithium batteries are reported (Solid State Ionics, 113-115, p57 (1998)) which use an electrode consisting of aluminum, silicon, tin or the like that is electrochemically alloyed with lithium on charge. Among these, a silicon electrode provides a particularly high theoretical capacity and is promising as a high-capacity negative electrode. For this reason, various rechargeable batteries using silicon for the negative electrode are proposed (Japanese Patent Laid-Open No. Hei 10-255768). However, such alloying negative electrodes fail to provide sufficient cycle characteristics since alloys, as electrode active materials, are themselves pulverized on charge and discharge to reduce current-collecting capabilities.
As a rechargeable lithium battery which uses silicon for the electrode active material and exhibits good charge-discharge cycle characteristic, the present applicant has proposed a rechargeable lithium battery which incorporates a microcrystalline or amorphous silicon thin film deposited on a current collector by a CVD, sputtering or other thin-film forming processes (Japanese Patent Laying-Open No. Hei 11-301646 and others).
It is an object of the present invention to provide a method for fabricating an electrode, for a rechargeable lithium battery, which uses a thin film of active material, such as a silicon thin film, and can provide a high charge-discharge capacity and good charge-discharge cycle characteristics.
A method for fabricating an electrode for a rechargeable lithium battery, in accordance with the present invention, includes depositing a thin film composed of active material capable of alloy formation with lithium on a current collector made of a metal incapable of alloy formation with lithium, using a process for depositing a thin film by supplying a material thereof from a gas phase, and is characterized in that the thin film of active material is deposited at such a temperature that enables formation, of a mixed layer via diffusion of a constituent of the current collector into the thin film in the vicinity of an interface therebetween.
Examples of processes that can deposit a thin film of active material by supplying the material from a gas phase include sputtering, CVD, vacuum evaporation and spraying processes.
In the present invention, any material can be used for the active material if it can form an alloy with lithium. Examples of such materials include silicon, germanium, tin, lead, zinc, magnesium, sodium, aluminum, gallium, indium and the like.
In view of the easiness of thin-film deposition by the aforementioned deposition method, the active material composed mainly of silicon or germanium is preferred. In view of the ability to provide a high charge-discharge capacity, the active material composed mainly of silicon is particularly preferred. Also preferably, the thin film of active material has the amorphous or microcrystalline form. Accordingly, an amorphous or microcrystalline silicon thin film is preferred as the thin film of active material. The thin film is identified as an amorphous silicon thin film when Raman spectroscopy detects the substantial absence of a peak around 520 cm-1 corresponding to a crystal region, and as a microcrystalline thin film when Raman spectroscopy detects the substantial presence of a peak around 520 cmxe2x88x921 corresponding to a crystalline region and a peak around 480 cmxe2x88x921 corresponding to an amorphous region. Other examples of preferred thin films include an amorphous germanium thin film, a microcrystalline germanium thin film, an amorphous silicon-germanium alloy thin film, and a microcrystalline silicon-germanium alloy thin film.
The current collector for use in the present invention is composed of a material incapable of alloy formation with lithium, such as copper.
In the present invention, the thin film of active material is deposited at such a temperature that enables formation of a mixed layer via diffusion of a constituent of the current collector into the thin film in the vicinity of an interface therebetween. That is, the diffusion of the current collector constituent into the thin film of active material is promoted as the temperature (thin-film forming temperature) at which the thin film of active material is deposited is increased. Accordingly, in the present invention, the thin film of active material is deposited at a temperature that enables sufficient diffusion of the current collector constituent into the thin film and sufficient formation, in the thin film, of the mixed layer consisting of the current collector constituent and the active material.
The formation of the mixed layer via diffusion of the current collector constituent into the thin film of active material improves adhesion of the thin film to the current collector. Also, the current collector constituent is a metal element which does not form an alloy with lithium. The diffusion of such a current collector constituent into the thin film of active material results in the relative reduction of expansion and shrinkage of the thin film of active material when it stores and releases lithium. A stress produced in the thin film of active material when it expands and shrinks is thus lowered in its location adjacent to the current collector. This prevents the thin film of active material, if its volume expands and shrinks, from separating from the current collector, and thus achieves further improvement of adhesion between the current collector and the thin film of active material.
In the mixed layer, the concentration of the current collector constituent in the thin film is found to be higher in the vicinity of an interface between the thin film and the current collector, and is lower at a location closer to the surface of the thin film of active material. This continuously decreasing concentration gradient of the current collector constituent in the mixed layer is considered to indicate the formation of a solid solution between the current collector constituent and the active material.
The higher thin film-forming temperature causes the excessive diffusion of the current collector constituent into the thin film and results in the increased tendency of the current collector constituent to form an intermetallic compound with the active material. The formation of such an intermetallic compound reduces the number of sites serving as the active material since the active material atoms are incorporated in the compound, so that a charge-discharge capacity of the thin film of active material is reduced. The formation of the intermetallic compound also reduces adhesion of the current collector to the thin film of active material. It is thus preferred that the thin film of active material is deposited on the current collector at such a temperature that does not produce, in the mixed layer, an intermetallic compound between the active material and the current collector constituent. Such a temperature is preferably below 300xc2x0 C.
In the present invention, a heat treatment may be performed after the thin film of active material is deposited on the current collector. The heat treatment allows further diffusion of the current collector constituent into the thin film. Hence, in the case where the mixed layer is formed to an insufficient thickness due to the failure to cause sufficient diffusion of the current collector constituent into the thin film during formation of the thin film, the practice of such a heat treatment is preferred. Preferably, the heat treatment is carried out under the conditions that avoid excessive diffusion of the current collector constituent and thus prevent formation of an intermetallic compound between the current collector constituent and the active material, as described above. A temperature for the heat treatment is preferably below 650xc2x0C., more preferably 400xc2x0 C. or lower.
In the present invention, the particularly preferred current collector constituent that diffuses into the thin film is copper. Preferably, at least a surface portion of the current collector is composed mainly of copper, since the copper diffuses from the surface portion of the current collector into the thin film.
In the present invention, the thin film of active material can be deposited by sputtering. In such an instance, a power density applied to a target containing constituent atoms of the active material is preferably 50 W/cm2 or lower, more preferably 6 W/cm2 or lower. The power may be supplied in any form, such as a DC, RF or pulse voltage.
Also in the present invention, the deposition of the thin film of active material is preferably effected in an intermittent fashion. The intermittent deposition of the thin film of active material is effective to lower a deposition temperature, i.e., a maximum temperature attained during deposition of the thin film. This therefore enables deposition of the active material under the conditions that the intermetallic compound is hardly produced. One method of achieving intermittent deposition of the active material on the current collector is to place the current collector on an outer periphery of a drum-like holder and deposit the thin film of active material on the current collector while rotating the holder.
The above-described process for depositing the thin film by supplying a material thereof from a gas phase is preferably practiced under the following conditions.
A substrate temperature is preferably below 300xc2x0 C., as described above. If the substrate temperature is excessively high, an intermetallic compound between the active material and the current collector constituent is occasionally formed.
The deposition rate is preferably 0.01 nm/sec (0.1 xc3x85/sec) or above. If the deposition rate is excessively low, the influence of surface diffusion and rearrangement becomes significant, even at low temperatures, to bring the process close to a thermal equilibrium, resulting in the increased tendency to form the intermetallic compound.
A pressure (degree of vacuum) of the atmosphere is preferably in the approximate range of 10xe2x88x922-102 Pa. If this atomospheric pressure (degree of vacuum) goes beyond the specified range, it becomes more likely that a thin film is provided as if formed by deposition of powder particles to result in the reduced adhesion thereof to the current collector. On the other hand, if the atomospheric pressure (degree of vacuum) falls below the specified range, the deposition rate becomes extremely slow to result in the increased tendency to produce the intermetallic compound, as described above.
As described earlier, when the thin film of active material is formed by sputtering, a power density applied to a target is preferably 50 W/cm2 or less, more preferably 6 W/cm2 or less. If the power density applied to the target is increased excessively, the influence of a radiation heat from a plasma becomes significant to result in the increased tendency of the active material to form the intermetallic compound.
The preferred sputtering gas is a gas which does not react with a target material such as silicon. From such a point of view, inert gases are preferred including He, Ne, Ar, Kr, Xe, Rn and the like. Among these gases, an Ar gas is particularly preferred for its ability to readily produce a plasma and provide a high sputtering efficiency.
A target for use in sputtering preferably has a single crystal or polycrystalline structure. Also preferably, its purity is at least 99%. These are to minimize inclusion of impurities in the resulting thin film of active material.
Preferably, an interior of a chamber before the start of thin-film deposition is maintained at a pressure of not exceeding 0.1 Pa. This is also effective to minimize inclusion of impurities in the resulting thin film of active material.
Before the deposition of the thin film, the current collector as the substrate is preferably subjected to a pretreatment, such as plasma irradiation. This plasma irradiation may be in the form of Ar or hydrogen plasma irradiation. The current collector can be cleaned at its surface by such a pretreatment. However, this pretreatment causes a temperature rise of the substrate. It is accordingly preferred that the substrate temperature is controlled to stay below 300xc2x0 C.
The current collector as the substrate may preferably be subjected to cleaning before the deposition of the thin film to clean the surface of the current collector. Examples of useful cleaning agents include water, organic solvents, acids, alkalines, neutral detergents and combinations thereof.
Where the heat treatment is performed after deposition of the thin film, the heat treatment is preferably effected at a temperature of 650xc2x0 C. or lower, more preferably 400xc2x0 C. or lower. At higher temperatures, the intermetallic compound may be produced, as described earlier.
Preferably, the thin film of active material is deposited onto the current collector in an intermittent manner. It is accordingly preferred that the current collector is placed on an outer periphery of a drum-like holder and the thin film is deposited on the current collector while rotating the holder, or the current collector is placed on a reciprocating holder and the thin film is intermittently deposited on the current collector. A possible alternative is to arrange plural targets and allow the current collector to pass through regions opposing the respective targets in a sequential manner so as to deposit the thin film intermittently. Such intermittent deposition of the thin film of active material suppresses a temperature rise of the substrate. The thickness of the thin film deposited each time in the intermittent deposition is preferably 1 xcexcm or less.